1. Field of the Invention
The present invention is related to the area of\LED (light-emitting diode) illumination, more particularly, to techniques for driving LEDs on multiple branches of a circuit.
2. Description of Related Art
In a prior art system, an LED drive IC with inductor-based step-up topology can drive only LEDs on one branch circuit, where all LEDs are serially connected to maintain the same current flowing through. In this case, the current difference among all LEDs is zero and current matching between LEDs is perfect.
Nowadays, with display screen size increasing, more and more LEDs are needed to provide uniform backlight for a LCD screen. Some conventional drive ICs are developed to drive WLEDs (white light-emitting diode) on several branch circuits by neglecting the difference of the forward-bias voltage between WLEDs, as shown in FIG. 1. If all WLEDs have exactly the same forward-bias voltage with the same current, the current through each branch circuit will be matched. Unfortunately, it is not the case in reality. With 3˜5 serially connected WLED in one branch, the total difference of voltage drop under a certain current will be big, sometimes up to 1V. It will lead to serious current mismatch between WLED branches if every branch is driven by the same voltage supply.
Some conventional design employs an internal current regulation loop to realize matching among LED branches as shown in FIG. 2. However, this current regulation circuit consumes additional power and leads to efficiency loss. To realize the matching, the difference of LED forward-bias voltage between two branches must be consumed on two NMOS field effect transistors MN1 or MN2. This power consumption will be transferred to heat. Sometimes this heat is so high and could cause IC reliability problems. Furthermore, this additional current regulation circuit introduces stability issues as well, needing much more design efforts.
Thus, improved techniques are needed for driving multiple branches of LEDs having matched currents going therethrough.